The present invention relates to improvements in electrostatic, xerographic and other types of electrophotographic copiers printers and, in particular, provides, as a substitute for the conventional wire, grid or mesh corona charging and discharging devices, a pile fabric brush device which will provide substantial charge uniformity in the charging and charge dissipating stations of such a copier printers and yet will perform at substantially lower voltages than has heretofore been the case without contacting the photoconductive surface. A second advantage is that the resultant distributed electrostatic charge is of a more uniform nature than the charge which is placed by current devices.
The essentials of the xerographic process are taught in U.S. Pat. No. 2,297,691, to C. F. Carlson. In general, the process involves the steps of placing a uniform electrostatic charge on a photoconductive insulating surface, exposing the surface to visible light and a shadow image to dissipate the charge selectively on the areas of the surface exposed to the light and then developing the resulting electrostatic latent image by the deposition on the surface of a developing material such as a refined electroscopic powder material. The particles of the powder will normally be attracted to the areas on the surface which retain an electrostatic charge to thereby form an image corresponding to the electrostatic latent image. Subsequently, the powder image is transferred to a receiving member such as a sheet of paper where the image is fixed as by fusing.
Essential to the success of the electrostatic copying process is the imposition of a uniform electrostatic charge on the photoconductive surface. To this end, the prior art has, in general, utilized corona charging devices in the form of one or more wires suspended across the photoconductive surface and which are connected to a potential voltage source to provide a potential difference of several thousand volts to efficiently create a net charge on the photoconductive surface. However, as recognized in U.S. Pat. No. 4,197,331, these devices are expensive, complex and potentially hazardous in view of the high potential power supplies required and the high concentration of ozone generated which will support spontaneous combustion or corrode other equipment parts in close proximity. For example, in order to charge a photoconductive surface to a potential of several hundred volts, it is necessary to impose a voltage difference between the photoconductive surface and the corona discharge device of several thousand volts in order to achieve a satisfactory uniformity of charge on the photoconductive surface.
Similarly, in high speed electrostatic copiers printers where the photoconductive surface is moved past a plurality of stations for immediate re-use, it is necessary to discharge the photoconductive surface and, again, it has been conventional to use wire type corona discharging devices connected to a potential source of different polarity to render the photoconductive surface ready for re-charging for subsequent copies and for facilitating cleaning of any residual toner from the photoconductive surface.
Also, with the use of such high potential voltages, the creation of ozone gas is inevitable and which is undesirable since this gas is highly corrosive.
Prior patents repesentative of the art in this field include: Nos. 2,790,082, 2,885,556, 2,952,241, 2,965,481, 2,968,552, 3,146,688, 3,223,548, 3,244,083, 3,332,396, 3,471,695, 3,866,572, 3,997,688, 4,122,210 and 4,164,372.
Of particular interest in U.S. Pat. No. 3,146,385 to Carlson, which discloses a contact charging apparatus for use in xerography which differs from the present invention in that the wires contact the photoconductive surface, on the one hand, and, on the other, in each embodiment, the wires are insulated from each other unlike the pile fabric of the present invention.
In U.S. Pat. No. 2,774,921, to Walkup, there is disclosed a charging device for a photoconductive surface where, in one embodiment, a pliable element is provided with bristles which are maintained in contact with the photoconductive surface. The pliable element is connected to a potential source that is described as lower than that usually used with conventional corona discharge devices. In another embodiment, the pliable element is used to charge the photoconductive surface without the bristles, and in this arrangement, the pliable element can be flexed against the photoconductive surface or spaced above and out of contact with this surface. Satisfactory operation was said to be obtained with this device where the elements have a resistance of between 10,000 ohms to about 100 megohms. Thus, highly conductive elements such as copper, silver and other common metals are disclosed as being unsuitable for the charging element.
The present invention provides a non-contact charging and discharging device for a copying apparatus which includes a brush-like structure of densely packed fibers of substantially uniform height where the fibers themselves are highly conductive, have minimal resistance and are each connected to a conductive base which in turn is connected to a potential voltage source of desired polarity or to ground. In one embodiment, the device can be used to charge a photoconductive surface at a charging station in an electrostatic copier printer and, in another embodiment, at a discharging station to dissipate electric charge and as a device for charging a sheet or photoreceptor which is to receive the copy to improve adherence of the developing powder to a latent image before and between the transfer station and fusing station of the apparatus. With this arrangement, much lower potential voltages can be employed while achieving substantially more uniform charge distribution and dissipation on the photoconductive surface because all the current used is useful in that it goes toward applying the desired charge to the photoreceptor surface. The current is not lost to a grounded shield or other screening device as found on current charging devices.